The present invention relates to a heat-resistant liquid separation membrane module having a semipermeable membrane receiving a pressure of a feed liquid and a channel material supporting the back side of the semipermeable membrane, where the heat-resistant liquid separation membrane module is enabled to treat a feed liquid at a hot or high temperature, and a method of producing the same.
A liquid separation membrane module using a semipermeable membrane is typically represented by a spiral wound type, in which a semipermeable membrane is formed into a long envelope (bag form). Into the envelope, a channel material is inserted that becomes a flow channel supporting a pressure of a feed liquid applied from the side of the semipermeable membrane and a permeate is introduced. The envelope is fixed to a hollow shaft at its open end and wound around it spirally with high density. In any of such a liquid separation membrane module, a feed liquid with a high pressure of not lower than the reverse osmotic pressure of the membrane is passed through the outside of the envelope, and the liquid permeated through the membrane is taken up through the inside of the envelope. Because the envelope receives a high pressure from outside, the channel material that is inserted as a flow channel for permeate is crushed, and thus liquid flow is deteriorated. Thus, in general, the channel material is rigidified so that it can resist deformation and so that the channel material is not crushed even if a pressure is applied from outside of the envelope. Such a liquid separation membrane module is used in a wide variety of applications, such as pretreatment of boiler water, reuse of waste water, generation of fresh water from sea water, and RO packaged units for generating extrapure water, etc. Water having a temperature of not higher than 40xc2x0 C. is used.
Traditionally, porous fabrics having fine channels extending therein, such as woven fabrics or knitted fabrics, have been used for this channel material, and particularly, a fabric having channels on its surface has been employed. Such a fabric has been rigidified by impregnating it with an epoxy or melamine resin, etc. so that it is not deformed easily by the pressure of a feed liquid applied via the membrane. To satisfy this, it was required that a resin is adhered to the fabric in an amount nearly half of its weight. However, in applications which requires a permeate with high purity or which processes a liquid of high temperature, problems have been caused by the elution of impregnated resins. Particularly, when the feed liquid to be treated with the membrane module is a fluid for food or medicine, it must be germ-free. Thus, the membrane module may be sterilized with a hot water to prevent contamination by germs of various sorts before or after carrying out membrane separation. Alternatively, in order to prevent contamination or control viscosity, or to prevent crystallization, a feed liquid itself may be treated with a high temperature greater than 40xc2x0 C.
In order to solve the above-mentioned problems, a channel material produced by a tricot knitting machine with three reeds, in which the fiber for forming a convex portion of the knitted fabric is thicker than the fiber for forming a ground stitch, and a melt adhesive fiber is knitted to rigidify the entire knitted fabric, has been proposed (Japanese Published Examined Patent Application (Tokko) No. HEI 3-66008). However, because such a channel material used three reeds and also used a yarn with a large fineness (denier) and a yarn with a small fineness (denier), it resulted in problems of low productivity and increased cost. Particularly, when a conjugated yarn having a low melting point component and a high melting point component was used, increased cost could not be ignored. Moreover, the thickness of the channel material could not be reduced.
Furthermore, in order to separate a solution with a liquid separation membrane module using a reverse osmosis membrane, it is necessary to apply a pressure of not lower than the osmotic pressure of the solution. At the same time, a differential pressure of usually about 0.5 to 1 MPa is loaded on the feed side and the permeate side. Then, if a single tricot knitted fabric 31 as shown in FIG. 10 or a double tricot knitted fabric 32 as shown in FIG. 11 is used for the above-mentioned permeate channel material, the reverse osmosis membrane is caved into the parallel channels aligning on one or both sides of the channel material and blocks them, resulting in a problem of increased flow resistance in the channels. When operation is continued for a long time under a high pressure while the reverse osmosis membrane is caved into the parallel channels of the permeate channel material, the reverse osmosis membrane 33 in contact with the concave and convex surface of the single tricot knitted fabric 31 is deformed as shown in FIG. 12, so that the cavities through which permeate is passed are crushed, and sufficient performance cannot be obtained. When a double tricot knitted fabric 32 is used, the reverse osmosis membranes 33 and 34 are deformed as shown in FIG. 13, so that the cavities through which permeate is passed are crushed, and sufficient performance cannot be obtained.
In order to solve the above-mentioned conventional problems, it is a first object of one or more embodiments of the present invention to maintain the structure and rigidity of a channel material for a long time without increasing flow resistance in channels and without impairing productivity of permeate, and to provide a liquid separation membrane module incorporating a channel material that does not cause elution and has a small thickness at a low cost.
It is a second object of one or more embodiments of the present invention to provide a practical liquid separation membrane module in which increase in flow resistance in channels of a permeate channel material caused by caving of a reverse osmosis membrane into the channels and deformation of the reverse osmosis membrane are inhibited, so that the performance of the reverse osmosis membrane can be maintained.
In order to achieve the above-mentioned first object, one or more embodiments of the present invention provide a heat-resistant liquid separation membrane module including a semipermeable membrane receiving a pressure of a feed liquid, and a channel material arranged so as to support the back side of the semipermeable membrane, wherein the channel material is a tricot knitted fabric knitted by a tricot knitting machine with two reeds and has a ground stitch portion and a convex portion. The tricot knitted fabric includes a thermoplastic synthetic filament yarn that has a core/sheath type conjugated fiber having a high melting point polymer as a core component and a low melting point polymer as a sheath component. Thermoplastic synthetic filament yarns for forming the ground stitch portion and the convex portion have substantially the same fineness. Yarns in the tricot knitted fabric are bonded with one another by melt adhesion of the low melting point component to rigidify the entire knitted fabric, thus forming the channel material.
In the liquid separation membrane module, it is preferable that the fineness of the thermoplastic synthetic filament yarns for forming the ground stitch portion and the convex portion is in the range of 45 to 55 denier.
Furthermore, in the liquid separation membrane module, it is preferable that the sinker loops of the knitted loops formed by one reed become the ground stitch portion, and the needle loops of the knitted loops and the chain portion formed by the other reed become the convex portion in the tricot knitted fabric.
Next, in order to achieve the second object of the present invention, it is preferable that, in one or more embodiments, the second liquid separation membrane module of the present invention further includes a flat fabric B laminated on the convex portion of the tricot knitted fabric A in the liquid separation membrane module, each of the fabrics is rigidified, and the cavity formed from the concave portion of the fabric A and the flat fabric B forms a flow channel that is not caved by the pressure required for reverse osmosis.
Furthermore, in the liquid separation membrane module, it is preferable that two or three layers are laminated.
Furthermore, in the liquid separation membrane module, it is preferable that the fabric B is a woven fabric or a nonwoven fabric.
Furthermore, in the liquid separation membrane module, it is preferable that the fabric B comprises a core/sheath type conjugated fiber having a low melting point polymer as a sheath component and a high melting point polymer as a core component, the fiber being melt adhered and fixed.
Furthermore, in the liquid separation membrane module, it is preferable that the fabrics A and B are integrated by melt adhesion, bonding or sewing.
Furthermore, it is preferable that the liquid separation membrane module can resist a reverse osmotic pressure of higher than 0 kg/cm2 but not higher than 200 kg/cm2.
Furthermore, it is preferable that the liquid separation membrane module is a spiral wound membrane element, in which a membrane is formed into a bag form, a permeate channel material being placed inside of the bag form, and the membrane is wound around a permeate collecting tube so that one end of the inside communicates with the permeate collecting tube.
Next, one or more embodiments of the present invention provide a method of producing a liquid separation membrane module including a semipermeable membrane receiving a pressure of a feed liquid, and a channel material arranged so as to support the back side of the semipermeable membrane. The method includes using a thermoplastic synthetic filament yarn comprising a core/sheath type conjugated fiber having a high melting point polymer as a core component and a low melting point polymer as a sheath component; knitting a fabric having a ground stitch portion and a convex portion by a tricot knitting machine with two reeds, while using thermoplastic synthetic filament yarns with a substantially same fineness for forming the ground stitch portion and the convex portion; heat treating the fabric at a temperature of not lower than the melting point of the low melting point polymer of the conjugated fiber but lower than the softening point of the high melting point polymer after it is knitted, so that yarns in the tricot knitted fabric are melt adhered with one another and fixed to rigidify the entire knitted fabric, thus forming the channel material; and placing the channel material on the back side of the semipermeable membrane.
In one or more embodiments of the method, it is preferable that the fineness of the thermoplastic synthetic filament yarns for forming the ground stitch portion and the convex portion is in the range of 45 to 55 denier.
Furthermore, in one or more embodiments of the method, it is preferable that the sinker loops of the knitted loops formed by one reed become the ground stitch portion, and the needle loops of the knitted loops and the chain portion formed by the other reed become the convex portion in the tricot knitted fabric.